1. Field of the Invention
The present invention relates to optical systems and, in particular, to apparatus and methods for tuning optical amplifiers and for cooling optical waveguides.
2. Technical Background
There is considerable interest in the field of optics, particularly relating to the area of telecommunication systems. Optical fibers are the transmission medium of choice for handling the large volume of voice, video, and data signals that are communicated over both long distances and local networks. Much of the interest in this area has been spurred by the significant increase in communications traffic, which is due, at least in part, to the Internet. Consequently, much interest is directed to increasing the data handling capacities of the optical fibers that comprise the communications networks. It is also of interest to increase the capacities of currently installed optical fibers and thereby eliminate the need for installing additional optical fiber cables. One way of increasing the capacity of optical fibers is to increase the number of channels or signals that are transmitted through each optical fiber. However, there are several technical difficulties with increasing the number of channels.
One limitation is that only certain wavelengths of optical signals can be efficiently transmitted through an optical fiber. For a typical optical fiber, the most efficient wavelengths (i.e. those with low loss per unit length) are in the S-band, C-band and L-band which comprise wavelengths from approximately 1460 nm through 1630 nm. Signals having wavelengths outside of this range suffer increased signal loss and are therefore cannot be transmitted efficiently along optical fibers.
Another limitation is related to amplifying the signals in the S, C, and L-bands when the signals are transmitted for long distances. Although modern optical fibers have very low loss per unit length, they nonetheless require periodic amplification of the transmitted signal to ensure accurate reception at a receiver. Initially, electronic amplifiers were used to amplify these optical signals. Electronic amplifiers converted the optical or light signals into electrical signals, amplified the electrical signals, and then retransmitted the signals as light signals. This was an expensive and inefficient way to amplify optical signals.
More recently, erbium doped optical amplifiers have revolutionized optical telecommunications by providing all optical high-gain, low-noise amplification over many channels without the need for the costly electronic repeaters. Optical amplifiers comprise a length of optical waveguide or fiber that is doped with a suitable fluorescent material such as erbium ions or thulium ions. A pump light source is injected into the doped fiber to excite the fluorescent ions. When a light signal is transmitted through the doped fiber, the excited ions release their energy at the same wavelength as the signal and thereby produce a highly amplified signal.
Unfortunately, erbium doped fiber amplifiers are primarily useful only in the C-band and, to a lesser extent, in the L-band. As bandwidth demand increases, the need to amplify signals outside of the conventional erbium doped fiber amplifier (EDFA) range increases. In the quest to expand the bandwidth of optical amplifiers, the prior art has attempted to develop exotic doping materials to create an improved optical amplifier. The prior art has also experimented with various complex light pumping schemes to improve the performance of optical amplifiers.
Yet another challenge confronting the industry are the increasingly tough specifications for amplifiers. Gain shaping, gain flattening, and gain tilt compensation are becoming more and more important as ripple and bandwidth specifications get tougher and wider.
The continuing challenge is to find an apparatus and method to increase the bandwidth of optical amplifiers and shape and flatten the gain of amplifiers so that the transmission capacity of optical fibers can be increased.
To address the problems discussed above, the invention discloses an apparatus and method for thermally tuning an optical amplifier which comprises: an optical wave guide doped with a fluorescent material, a thermal device for either heating or cooling the optical wave guide, and a pump light source for exciting the fluorescent material.
It has been discovered that by changing or controlling the temperature of a doped optical amplifier, the gain curve of the fiber can be shifted or shaped over various wavelengths. Accordingly, the invention uses this discovery to shape, shift, and/or flatten the gain curves of doped optical amplifiers to achieve the desired results. For example, thulium doped optical fiber is cooled to shift the gain curve into longer, more useful wavelengths. Similarly, erbium doped optical fiber is cooled to shift the gain curve to longer wavelengths above the L-band. As yet another example, an erbium doped optical fiber is heated to flatten the gain curve in the C-band. The invention may also be used to shape the gain curves of other fluorescent materials used in optical amplifiers.
In order to achieve the cooling of the optical amplifiers, three optical cooling devices are disclosed. The first optical cooling device comprises an optical fiber (cooling fiber) doped with an appropriate fluorescent refrigerant material and coupled with a pump light source. The cooling fiber is co-wound with an amplifying fiber and the cooling fiber is cooled as the pump light is activated. The amplifying fiber is cooled by the cooling fiber due to their close proximity.
A second embodiment comprises doping the cladding of the optical amplifier fiber with a fluorescent refrigerant such as ytterbium. A pump light source is coupled with the cladding to cause the cladding to cool and thereby cause the core of the fiber amplifier to cool. An additional advantage of using this embodiment is that some fluorescent refrigerants, such as ytterbium, produce pump light as a by-product. This pump light may then be used to pump the fluorescent material in the core.
A third embodiment comprises an optical fiber amplifier having two cores. A first core is doped with a fluorescent material suitable for amplifying an optical signal. The second core is doped with a fluorescent refrigerant material. When pump the is applied to the second core, the core cools and also causes the nearby first core to cool.
The apparatus is useful in not only in telecommunication systems, but also in laser printers, laser marking, medical imaging, microsurgery, scientific research and development, and the like. It follows that the method of the invention comprises either heating or cooling an optical waveguide doped with fluorescent material to achieve desired shaping, shifting, and flattening of the gain curves.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.